[ovirt-devel] logging VDSM-generated libvirt XML properly

[Martin Polednik]

On 03/03/16 16:46 +0200, Nir Soffer wrote:
>On Thu, Mar 3, 2016 at 4:28 PM, Martin Polednik <mpolednik at redhat.com>
>wrote:
>
>> Hello developers!
>>
>> I've been chasing a bug that lead me to an idea of improving our XML
>> logging. Now, to see a VMs generated libvirt XML, we have to rely on
>> vdsm.log. The issue is that the log is rotating and therefore, it is
>> easy to miss the correct XML when dealing with busy hypervisor.
>>
>> Since we're built on libvirt, I was thinking of doing similar thinks
>> that libvirt does with qemu commandline. Each running domain(VM) has
>> it's command line logged in /var/log/libvirt/qemu/${vmname}. This is
>> great for debugging as you can mostly just take the cmdline and
>> restart the VM.
>>
>> There is an issue with using the cmdline directly - networking.
>> Libvirt uses additional script to create and up a bridge. Therefore,
>> it is easier to use the XML and shape it to one's needs.
>>
>> I propose that we properly log the generated XML in a similar fashion
>> as libvirt generates the cmdline. This means we would have path like
>> /var/log/vdsm/libvirt/${vmname}, where generated XML would be stored.
>> To minimize the logging requirements, only last definition of VM with
>> that name may be stored. Additionally, exception level errors
>> related to that VM could also be stored there.
>>
>
>Without history, it will much less useful, so I think we should keep this
>in vdsm log.

Since you mentioned the logging level, it makes sense to actually
include the history. Even without it, use case (having 'last run
XML available') is still valid.

>How about creating separate log for virt?

Would be better, but still doesn't solve ^.

>We talked about separating the application to several parts, so we can
>start by
>having separate logs. In /var/log/vdsm/virt.log (or /var/log/vdsm/vms.log)
>you
>will see calls to the storage layer (service in the future), but you will
>not see all
>the noise that storage generates (e.g. resourseManager crap).
>
>
>>
>> What do you think, can we afford the space and additional writes per
>> VM to help the debugging process?
>>
>
>We can, but the default log level should not show stuff like full xml dumps.
>
>Nir